One idea could be a rengoku fanfic where he trains a new group of young slayers. It could focus on his unique teaching methods and how he instills his values in them.
Tanjiro would be a great character to pair with rengoku. Their different personalities could create an interesting interaction. Tanjiro's kind - hearted nature and rengoku's fiery determination could lead to some great storylines.
One of the main themes is adventure. Since rengoku is often involved in battles and dangerous situations, the fanfics may take the reader along on these adventures. They might face demons together and their relationship develops through these shared experiences. Also, loyalty is a big theme. Rengoku is a very loyal character, and this loyalty extends to his relationship with the reader in the fanfics.
No, bondage fanfic is generally not appropriate as it often involves themes that are not suitable for public consumption and can be considered a form of non - consensual or extreme behavior in a fictional context.
You can try searching on popular fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfictions across various fandoms, so there's a good chance you'll find some great Ashe Basch fanfiction there.